Damage control: PostEntity now has validations.

Proper ones, not hand-crafted, half-baked and replicated all over.
(Well, we haven't ripped the replication out just yet. That's coming.)

RSpec: 773 examples, 0 failures
RuboCop: 178 files inspected, no offenses detected
Damage control cont'd: UserEntity has validations.

Now to rip the ad-hoc stuff out.

RSpec: 781 examples, 0 failures
RuboCop: 178 files inspected, no offenses detected